fix: show balances and fees below 0.000001 as nonzero on the send screens (closes #343)

The stored ETH and token balances and the send-confirm screen's fee were each
cut to six decimal places, and a token holding cut to zero was dropped, so a
value below 0.000001 read as zero. Balances are now stored exactly, whatever
decimals a token declares, and every nonzero token holding is kept; the balance
check reads a token balance to its first 18 places. The balance lists, the
send-screen token selector, the address total and the remove-address warning
leave out a holding below 0.000001 themselves, through isBelowOneMillionth().
The send and send-confirm screens' balances, reserve and insufficient-balance
messages go through truncateAmountNeverZero(). The send-confirm and approval
screens both render the fee through formatFee(), which prices the exact fee in
USD.

Model: opus-5-5

src/shared/prices.js

@@ -1,6 +1,7 @@
 // Price fetching with 5-minute cache, USD formatting, value aggregation.
 
 const { getTopTokenPrices } = require("./tokenList");
+const { isBelowOneMillionth } = require("./amountDisplay");
 
 const PRICE_CACHE_TTL = 300000; // 5 minutes
 
@@ -78,6 +79,9 @@ function getAddressValue(addr) {
     let usd = parseFloat(addr.balance || "0") * prices.ETH;
     let partial = false;
     for (const token of addr.tokenBalances || []) {
+        // A holding below 0.000001 is left out, as the balance lists leave it
+        // out, so the total never counts a holding the list does not show.
+        if (isBelowOneMillionth(token.balance)) continue;
         // A null balance is a holding whose scale nothing knows, so it has no
         // quantity to price — but it is still a holding, and a total that
         // silently omits it would read as complete. That is exactly what